The invention relates to a magnetic erase head formed by a permanent magnet, which comprises a permanent magnet material such as ferrite.
Various forms of magnetic erase heads are known and used in magnetic recording and reproducing apparatus, including
(A) D.C. TYPE IN WHICH EITHER THE COIL OF THE HEAD IS FED WITH A DIRECT CURRENT OR A PERMANENT MAGNET IS USED TO MAGNETIZE A MAGNETIC MEDIUM TO ITS SATURATION;
(B) A.C. TYPE IN WHICH THE COIL OF THE HEAD IS FED WITH A CURRENT OF A FREQUENCY WHICH IS SEVERAL TIMES HIGHER THAN THE MAXIMUM FREQUENCY RECORDED TO REDUCE THE REMANENT FLUX IN THE MAGNETIC MEDIUM SUBSTANTIALLY TO A ZERO LEVEL;
(C) THE USE OF A PERMANENT MAGNET MATERIAL WHICH IS MAGNETIZED TO PRODUCE ALTERNATE POLES OF OPPOSITE POLARITIES SO THAT AT THE END OF TRAVEL OF THE MAGNETIC MEDIUM ACROSS THE POLES, THE FIELD STRENGTH OF THE MAGNET CONVERGES TO ZERO, ACHIEVING AN EFFECT SIMILAR TO THE A.C. ERASURE; AND
(D) AN INITIAL ERASURE BY AN ERASE HEAD TO THE SATURATION, FOLLOWED BY THE APPLICATION OF A D.C. FIELD OF THE OPPOSITE POLARITY BY A RECORD HEAD TO COMPENSATE FOR THE OFFSET CREATED BY THE SATURATION EFFECT, THUS ACHIEVING SUBSTANTIALLY THE SAME EFFECT AS THE A.C. ERASURE.
The technique (a) suffers from large erasing noises and distortion, and the technique (b) involves an increased power dissipation, which renders it inadequate for use in a tape recorder of portable type. The technique (c) requires the provision of five or seven poles, resulting in a bulky construction. Additionally, their manufacture is expensive in that the successive poles must be magnetized to sequentially decreasing levels. Finally, the technique (d) employs a head which is also used for record and playback operation and having a small gap length on the order of one micron, which may cause an increased distortion when the d.c. current is applied to the head.
In a so-called miniature cassette tape recorder which is designed to be received in a pocket of a shirt, the erase head has a width which is 2.1mm at maximum the tape cassette is comparable in size to a packet of matches. The contact between the head and the tape can only be maintained over a length of 0.8 mm. With such a recorder, the available erase head is dimensionally limited to the types (a) and (d).